What's stopping someone from just making another mineable coin? They will call it Derpthereum and everyone will completely lose their minds and start selling their houses, harvesting their children's organs, robbing banks etc just to get GPU money so they can mine.
This all depends on there being an actual market for that coin. There are a ton of coins that almost no one wants to have and are thus quite worthless.
Mining of a specific coin is only profitable for the medium term if:
- People actually want to pay decent money for what you mine
- The difficulty of mining is low enough that mining earns more than it costs.
New coins with little demand fail on both. As soon as you start mining them in quantity, the market is flooded with new coins way beyond demand, so prices will fall a lot. Also, the difficulty goes up really fast for coins that haven't been mined that much, so you very quickly start to mine far fewer coins for the same expenditure. So the combined effect is that profitability of mining will fall like a stone.
Bitcoin and Ethereum are a bit special in that the demand is large and kept growing, so mining kept being profitable for a long time (although ASIC mining outcompeted GPU mining for Bitcoin and Ethereum profitability has deteriorated a lot already). Even without proof of stake, mining for Ethereum can't be highly profitable forever, as it would require a permanent growth in demand for Ethereum.